Printing machines, particularly sheet-fed offset printing machines, utilize drives, often in the form of direct current motors, to drive via a continuous gear train both the cylinders of the printing units and the cylinders or drums used in transporting printed material. Due to the large number of printing units in a printing machine, very high load torques are produced in the gear train which, in turn, produces torsion between the individual printing units. The resultant torsion causes doubling phenomena and registration differences to occur as a result of changes in the load on the gear train.
Furthermore, conventional sheet-fed offset printing machines that have a feed-in point driven by a drive motor require complicated mechanical components in order to change the mode of operation from face-side printing to perfecting and vice versa. A disadvantage in printing machines driven by a continuous gear train is that automated operations in the individual printing units must be carried out one after another because the individual processes require different directions of rotation and/or different rotation speeds. For example, automatic washing of blanket cylinders or printing cylinders is carried out separate from changing the printing forms or printing plates.
Disclosed in U.S. Pat. No. 5,377,589 (and corresponding German Patent DE 42 41 807 A1) is a drive for a printing press in which the effect of the drive train on synchronization is reduced for those elements of the printing machine which are not involved in transporting the printed material. According to the device, a first drive, which includes one or more motors, is provided for all cylinders serving the transport of printed material and for the plate cylinders which are connected to one another via a continuous gear train. The elements not serving the transport of printed material, such as the inking or damping solution units, are attached to separate drives which are executed in a position controlled manner in relation to operation of the cylinders. Synchronization of these mutually decoupled partial systems is accomplished by means of signals from sensors detecting movement of the various components and cylinders. Although the drive avoids some of the negative effects of torsion on the printing process, the plate cylinder of one printing unit is mechanically coupled via the continuous gear train to the other printing unit cylinders which results in load fluctuations being transferred directly to the plate cylinder. These load fluctuations, in turn, cause printing disturbances.
Disclosed in U.S. Pat. No. 5,309,834 (and corresponding German Patent DE 42 14 394 A1) is a rotary printing machine having a number of directly driven cylinders and at least one directly driven folding unit, wherein the individual drives of the cylinders and their drive controllers are combined to form printing-station groups. The printing-station groups, which are connected to one another and assigned to one of the folding units, receive their position reference from the folding unit. Management of the printing-station groups is performed by a high order control system. Applying this previously known solution to a sheet-fed offset printing machine allows each individual cylinder, including printing unit cylinders and cylinders or drums used for transporting printed material, to have a dedicated drive whose position can be controlled. While increased functionality is achieved, in particular for implementation of automated operations, the number of drives required to implement the configuration becomes highly cost-intensive. Furthermore, precautions must be taken to ensure that the sheet-guiding cylinders having gripper devices are capable of rotating relative to one another to the maximum permissible angular amount. Otherwise, damage will occur to the equipment of the gripper systems projecting away from the circumference of the cylinder.
Disclosed in U.S. Pat. No. 4,980,623 (and corresponding European Patent EP 0 355 442 B1) is a method and device for reducing torque loading on a system driven by means of an electric motor. According to the invention, changes in the load torque are counteracted by oppositely directed changes in the drive torque such that the load torque is maintained at an approximate constant value. Combining this method and device into a sheet-fed offset machine having a plurality of printing units, however, only minimizes the torsion produced by the load fluctuations and produces no gain in functionality.
Disclosed in German Patent DE 42 41 807 A1 is a method and drive for a printing machine having a plurality of printing units in which a plurality of drive motors drive a continuous gear train. According to the invention, a first drive motor feeds an excess of power into the drive train. The excess is proportioned to ensure a constant direction of power flow in the drive train. A final drive motor compensates for the excess power flow and regulates power flow through the entire drive train. In sheet-fed offset printing machines, use of this method and corresponding drive avoids tooth flank changes which result in uncontrollable stressing. The disadvantage of this method is that the angular position of the feed motors or of the cylinders driven by the motors is undefined resulting in an angular position strongly dependent on the phase and tolerance of the gear wheels.